Use config.cache variable for PIC flag, to allow override.

Use config.cache variable for PIC flag, to allow override.

* libltdl/m4/libtool.m4 (_LT_COMPILER_PIC) <lt_prog_compiler_pic>:
Save result, and allow override, through cache variable
lt_cv_prog_compiler_pic{,_CXX,_F77,_FC}.  The cache variable
needs to contain an eventual -DPIC.
* doc/libtool.texi (): Document it.
* tests/pic_flag.at (override pic_flag at configure time):
New file, new test.
* Makefile.am (TESTSUITE_AT): Update.
* NEWS, THANKS: Update.
Suggestion by Joakim Tjernlund and Paolo Bonzini.

Signed-off-by: Ralf Wildenhues <Ralf.Wildenhues@gmx.de>